In general mobile communication systems, since a density of subscribers is different at every region and at every time, the tilt control is frequently required in order to optimize the mobile communication system. For optimization of the air interface network, in a conventional mobile communication system, a mechanical tilt is used. The beam tilt of the antenna in a vertical direction means an angle of the beam radiated by the antenna slopes to the horizontal.
A conventional antenna is mechanically tilted to vary the radiated beam tilt of the antenna, using a mechanical tilting device mounted on the antenna.
Mechanical tilting of the antenna is a cost-effective way to manufacture the antenna. However, in this case personnel have to climb up toward the antenna to manually adjust antenna beam tilt. It is neither economically viable nor time-conscious. In other words, when the beam tilt of the antenna is required, the person should climb up toward the antenna, unfasten bolts fixing the tilting apparatus, adjust the angle of the antenna, and fasten the bolts, which takes much more time to tilt the antenna.
To solve the abovementioned problem, an electric beam tilting device capable of adjusting antenna beam tilt at a distance is developed. Such electric beam tilting device includes a phase shifter for shifting a phase of the beam radiated by the antenna.
A phase shifter for adjusting antenna beam tilt is disclosed in Korean Patent Laid-open No. 2002-0041609 which describes the phase shifter in which the beam tilt is varied by both adjusting the phase of the radio waves radiated by the antenna and controlling the power division.
FIG. 1 is a view showing a conventional phase shifter.
As shown, the conventional phase shifter includes a power divider 51, a first phase shift unit 52, a second phase shift unit 53, a first delay unit 54 and a second delay unit 55.
A radio signal is fed into the power divider 51 via an input port (IP). The power divider 51 divides up the radio signal in a predetermined ratio and then feeds them into the first and second phase shift units 52 and 53. The first phase shift unit 52 adjusts the phase of the radio signal and then sends it out to both a first output port (OP3) and a second output port (OP4). The second phase shift unit 53 divides the radio signal into two separate parts moving away in opposite directions to obtain phase shifts between them. The first and second delay units 54 and 55 are electrically connected to the second phase shift unit 53, facing each other. On the one hand, the first delay unit 54 delays the radio signal and then pass the delayed radio signal on to a third output port (OP5). On the other hand, the second delay unit 55 delays the radio signal and then send them out to a fourth output port (OP6). Ideally, the phase difference between output signals at the OP5 and the OP6 is constant.
When the power divider 51 divides the radio signal into two parts in the ratio of 1 to 2, the intensity of one part fed into the second phase shift 53 is two times stronger than that of the other part fed into the first phase shift units 52. That is, one part of the power divider 51 has a degree of zero and an amplitude of 1 (i.e., 1<0). The other part of the power divider 52 has a zero degree and an amplitude of 2 (i.e., 2<0).
The radius of a circular shape formed by the microstrip transmission line making up the first phase shift units 52 is roughly 3 times larger than that of the second phase shift units 53. If the phase of the radio signal received via the IP is not changed, The output signals at the OP3, OP5, OP6 and OP4 are outputted at the same time.
When the first and second phase shift units 52 and 53 are rotated by certain degrees, the phase difference between input and output signals at the OP3, OP5, OP6 and OP4 are 3 Θ/2 , Θ/2, −Θ/2 and −3 Θ/2 respectively. That is, the OP3 has a degree of +3 Θ/2 and an amplitude of 0.5. The OP5 has a degree of +Θ/2 and an amplitude of 1. The OP6 has a degree of −Θ/2 and an amplitude of 1. The OP4 has a degree of −3 Θ/2 and an amplitude of 0.5. In this case, the phases of the adjacent output signals differ by Θ.
Following from the above, the function of the first and second phase shift units 52 and 53 is to vary the phase of the radio signal fed into the antenna via the OP3 and OP6, thereby varying its power distribution.
However, the main drawback to the conventional phase shifter is that there is a need for an additional power divider capable of acquiring an output signal that has the same phase as the input signal. In addition, as the phase shift units are turned by certain degrees to vary the phase of the input signal, the radio signal fed into a metallic contact between a fixed part and a variant part is likely to go through an intermodulation. In this case, attainable variation in the angle of antenna beam tilt in vertical directions is limited largely due to a one-dimensional way the delay units delay the radio signal. Here, the delaying of the radio signal is done by making use of the distance between the radio signals.